An imposing celestial body with a radius of 555 kilometers from the regions of space located beyond the orbit of Neptune, Quaoar is known to have briefly held, in 2002, the record for the largest object in the Solar System discovered since Pluto (1 186 km radius). According to the work presented in review Nature from February 2, he will now be able to take advantage of another particularity: that of possessing the most bizarre ring ever spotted. Bruno Morgado, from the Federal University of Rio de Janeiro, Bruno Sicardy, from the Observatoire de Paris-PSL, and their colleagues succeeded in detecting this disc of debris using the method of stellar occultations. Above all, they characterized it with enough precision to call into question the theories in force on the conditions of formation of this type of structure.
As one of the wonders of the Solar System, the planetary rings do not, strictly speaking, correspond to objects. But to areas of space occupied by a myriad of fragments of all sizes. From the residue of a few centimeters to the small moon. If Saturn’s disc is the most famous of all, other crowns, less wide, less dense or made of darker material, also surround, completely or partially, Jupiter, Uranus and Neptune. And the same would be true, we learned, in 2014 and 2017, of certain asteroids and planetoids, such as Chariklo and Hauméa.
Do these entities date from the time of the formation of the planets of the Solar System? Or are they the later result of a collision with an external object or the destruction of a moon? Astronomers do not risk proposing a scenario. But note that all these discs orbit at short distances from their celestial body, below the “Roche limit” – named after the French mathematician and astronomer Edouard Roche (1820-1883) -, which determines the region where the action of the tidal forces exerted by an object is sufficient to dislocate a satellite.
Observing Stellar Occultations
The corona spotted by the group of Bruno Morgado and Bruno Sicardy around the planetoid Quaoar is well beyond this limit. A discovery made thanks to the observation of stellar occultations. This proven astronomy process makes it possible to gather information on bodies in the Solar System that are too small or too far away to be easily studied by other means. The dimensions, the shape, the possible existence of a ring or an atmosphere are made accessible simply by analyzing or timing the drop in luminosity caused by the passage of these objects in front of a star. At least, when we manage to detect the phenomenon, which is not always possible because of uncertainties about the position and movement of the stars. A problem well known to amateurs and professionals, often faced with the difficulty of predicting in what narrow strip of the surface of the terrestrial globe – its width must be equal to the diameter of the object studied – they will need to deploy their equipment.
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